W7IEQ Power Meter Design & Build

[G0MGX]

So, progress is slow and steady today.... Here is my next dumb question:

How do I test a directional coupler?

If I connect a radio to the coupler and then to a dummy load, then terminate the forward and reflected ports with 50R, I should see N volts p-to-p on the forward port and not much on the reflected port - am I correct?

Mark. G0MGX

[G3RZP]

yes.

Then turn it round and see that you get the same effect. Of course, Forward and Reflected will reverse too.

[M0HCN]

I would build the coupler in a box made by soldering bits of bare PCB together, it being easier to get good ground connections that way then building in a diecast ally box.

Use double sided board and put a little copper foil across the edge of the boards before soldering the lid on (You cannot directly solder the interior seam of the lid!).

Regards, Dan.

[W8JI]

Mark,

I usually use double sided PC board for boxes that demand good RF bonding.

You can use die cast or aluminum, but it requires many holes, machine screws, and lugs.

That's an interesting, but rather odd, directional coupler. I see what they are doing, and it requires the current transformers be terminated in something that matches the interconnecting cables. The problem is optimum transformer flatness might not be 50 or 75 ohms, whichever you use.

Then again, it might be OK. 

I think I'll do a search and see what the thing is.

73 Tom

 

[G0MGX]

All good, I've plenty of double sided copper board, no idea why I didnt think of that myself!

Can anyone point me at a picture or diagram of a built more conventional directional coupler? Its not something I have played with or looked into before - a visual reference would be veryu useful.

Progress will slow now as I have to go and do stuff for my employer....

Mark. G0MGX

[KA4POL]

http://www.dj4uf.de/projekt/swr/swr.html
Google will help you translate. The pictures are pretty self explanatory.

[G0MGX]

Well, all very interesting.

I found some time this evening and have remade the (already remade) directional coupler. Here's take 3:

http://www.qsl.net/g0mgx/images/DSC_0082.JPG

So this model features:

• FT-82-76 Toroids
• Home bodged double sided copper box with one partition
• Shorter leads
• About 20 miles of solder consumption to make a good screened enclosure

I've taken a HF radio in FSK mode, connected it to the coupler and then connected the coupler to a dummy load. The forward and reflected ports of the coupler are connected to the AD8307 boards which have a 52.3R input impedence, across each of these I have a x10 scope probe, here is the result NOTE the Ch2 voltage needs to be divided by 10 due to my cockup in scope config:

http://www.qsl.net/g0mgx/images/scope.png

So I have 20W out of the rig, 2.4 (ish) p-to-p out of the forward port and about 50mv (ish) out of the reflected port. This seems a good place to start. However, the meter design takes the p-to-p value through the AD8307 then into the sample and hold LF389 and then directly into the A to D converter in the PIC. My 20W is now being displayed as 444W!

2.4V p-to-p for a 20W RF source is WAY to high! The A to D converter is maxing out at about 30 Watts or so. The A to D converter is configured in the 16F876 to be using AD3 (thats connected to a 2.5V reference) as the +ve reference and 0V as the negative reference. So the analogue input is being compared to a 0 to 2.5V range.

The other thoughts that I have right now is that the software contains a whole pile of config tables which are based on a evaluation of the voltage presented to the A to D converters from the two channels under different known RF power levels of drive. The chances of that config being remotely accurate in my build of the meter seem to be rather slim (like nill) to me.

So I have some good progress, but the voltage out of the directional coupler seems too high to me; I'll worry about other stuff like calibration later!

Mark. G0MGX

[M0HCN]

Check that the number of turns on both toroids are equal, for a toroid a turn is counted each time a wire passes through the hole.
That coupler is good, but actually still physically too large, and I usually run the screen the other way with a pair of insulated feedthrus for the voltage and current transformer connections.

Usually you need a 50 ohm pad between the head and the sensing electronics, the log amps are usually only good for 0dBm or so, and 20dB of pad will also improve the match to the coupler.

10 log (444/20) = 13.5dB, so maybe a simple 10dB pad and trim out the last factor of two with the dc offset trim on the log amps.

Regards, Dan.

[G0MGX]

Finally I have some progress to report, not quite the way you might expect.

I've just updated my blog, so here's a link to save duplication here:

http://g0mgx.blogspot.co.uk/2012/11/the-power-is-with-me.html

Mark. G0MGX 

[KA4POL]

That's worth publishing in QST